The present invention relates generally to a process for artificially producing a high quality, high density abrasive "novaculite" sharpening stone which closely resembles naturally occurring Washita stone. More particularly, the present invention relates to improved processes for producing an artificial, tinted novaculite Washita stone. The invention is believed best classified in U.S. Class 51, Subclass 293.
As explained in detail in our above referenced patent application, novaculite is a form of quartz which itself is primarily comprised of silicon dioxide. In its natural form novaculite has been mined, particularly in Arkanas, for use as a sharpening stone, and it has been known in the trade by the misnomer "Washita." It is increasingly becoming rare. Naturally occurring novaculite typically exhibits density of approximately 2.60 in molecular form, 2.3 in stone form and a MOH hardness of between 7-7.5, with natural stone of 7.5 MOH hardness being very rare.
At the present rate of consumption in domestic cutting shops it is likely that presently known deposits of high grade novaculite will be consumed in the next thirty years. One reason for this is that conventional cutting processes inevitably waste approximately 75% of all quarried stone, in order to arrive at an end product stone which minimizes flaws and which is suitable for honing.
A typical sharpening stone cut from naturally occurring novaculite suffers from several imperfections. First, naturally occurring novaculite is not homogenous and regions of different density exist in the stone. Fissures such as quartz intrusions, cleavage or the like exist along certain fault lines which weaken the stone, and which indicate a region of differential density or sharpening characteristics. Natural stone may experience such deleterious fissures through natural processes when subject to freezing during the winter, and as a result most quarries must mine suitable novaculite at a substantial distance beneath the ground. This further aggravates the present shortage of high grade Novaculite.
Other imperfections which are often associated with mined novaculite Washita stone include sand pits, fault lines, incursions, and quartz seams. Incursions are minor imperfections in the surface such as sand pits which are formed when the stone is cut through and portions of sand naturally formed within the item being cut are exposed. Such sand pits continue to widen and enlarge as time goes on so that such a conventional finished stone product, when used as a sharpener, degrades. Natural novaculite also exhibits substantial color and density difference from sample to sample, which may deleteriously effect the aesthetic value of a finished stone. As explained in our above identified copending application, naturally occurring novaculite subject to the aforementioned difficulties may further interfere with proper knife sharpening.
Notwithstanding the foregoing, natural Washita stone has several attributes when compared to some forms of Novaculite, Washita stone is coarser than Arkansas hard white stone, for example. It is characterized by less density (i.e. 5% less dense than Arkansas hard white stone), and it is both softer and of less tensile strength. Hence it is easier to machine and form. Washita stone also produces a more aggressive honing action than other forms of Novaculite. Because there are more connecting voids overall in the Washita stone, as quarried, and because of the larger particle size and reduced characteristic hardness, Washita stone is often more suited for general sharpening purposes.
It will also be appreciated that natural Washita stone is often characterized by distinctive and aesthetically pleasing color striations or patterns often beautifully interspersed with multicolored swirls and layers. Such color patterns are especially enhanced by flat face cutting and lapping which shapes the stone for its use as a sharpening hone. It is suggested that the characteristic color patterns of Washita stone result from contamination from metal oxides diffused through the product in the heat of formation. However, as explained in our copending parent patent application, the surface color zones in mined novaculite stone often coincide with regions of cleavage or weakness.
It is desirable to provide a system for artificially manufacturing "striated" Washita novaculite exhibiting aesthetically attractive characteristics, while as the same time minimizing the above discussed weaknesses. Thus, for example, a process for creating an artificial Washita stone without contaminant sand pits, fault lines, incursions, and quartz seams is warranted. Moreover, it is advantageous to produce an artificial Washita novaculite stone which exhibits substantially homogenous density so as to consistently either "polish" or "grind" as desired by the user. Moreover, it is desirable to produce an appealing tinted, Washita stone without cleavage or fissures which shorten the life expectancy of prior art stones cut from naturally occurring novaculite deposits.
Perhaps one of the earliest patents relating to this art is U.S. Pat. No. 1,983,082 issued to W. L. Howe on Dec. 4, 1934. The latter reference describes how raw materials of granular abrasive material may be formulated with a vitrified bond after a mixing process. The bond is matured after pressing and shaping by heating until vitrification occurs, at which point cementing of the various constituent grain occurs. Such a bond results in regions of voids, between which the "grinding" abrasive characteristics of the resultant stone are produced. Other prior art is seen in the following U.S. Pat. Nos. 2,880,081, 3,003,860, 3,092,476; 3,142,138; and, 4,541,843.
However, no process known to us has hitherto yielded artificially produced Washita type novaculite exhibiting superior color qualities, homogeniety, uniform density, high strength (both tensile and abrasive), high MOH hardness in excess of MOH 7.0, and virtually complete omission of fissures, contaminating incursions and the like.